What is human hair?A light and scanning electron microscopy study

 

Xue Wei

Department of Mechanical Engineering, University of Rocheste, NY 14627

May 2, 2006

 

Introduction

As shown in Fig. 1, human hair fibers have various layers of cellular structure, . they (about 50-100 um in dameter) consist of cuticle and cortex, and in some cases medulla in the cnetral region. All of them are composed of dead cells, which are mainly filled with keratin protein. The keratin is characterized by a high content of cystine, an amino acid that has the capacity to cross-link the protein by its intermolecular disulfide linkages. A high cystine content corresponds to rich disulfide cross-links, leading to high mechanical properties. In addtiton to disulfide bonds, hair is also rich in peptide bonds and the abundant CO-and NH- groups present give rise to hydrogen bonds between groups of neighboring chain molecules.

Fig. 1 Schematic of hair fiber structure, and cross-section of human hair fibers

Human hair is categorized into 3 major distinct groups according to enthnic orgin: Asian, Caucasian, and African. the aims of this study are: (1) to see the difference characteristics of these three kind hair (2) to investigate the effect of a common conditioner on human hair.

Hair samples

The hair samples used in this study were cut from three girls, who come from Chian, America, and Guyana repectively. When investigate the influence of conditioner, the hair was immersed in the conditioner for one day. Before any measurement, hair samples were washed with the same shampoo, rinsed and aire dried. Before doing SEM, the hair samples were coated with a thin layer of gold by sputtering.

Part 1 Characteristics of various hair

Fig 2. Cross-section of various hair

Diameter and section: Aaian hair has the greatest diamter with circular geometry while African and Caucasian hair shows an elliptic section.

Fig .3 SEM images of various hair

Asian Caucasian African

 

Fig .4 Light microscopy of various hair

Morphology : The basic structure of the hair shafts consisting of a cortex enclosed by a multiple layer of overlapping cuticular cells is similar for all three groups. African hair has extremely curly configuration.

 

Fig .5 X-ray spectroscopy of various hair

 

Chemical composition: Proteins and amino acid constituting keratin are similar in African, Asian, and Caucasian hair as shown in the above x-ray spectrometry. Compare the relative amout of sulfur, we found the more sulfur the more straight of the hair. This result is consistent with the introduction. The rich disulfide cross-links lead to high mechanical properties.

 

Part 2 Effect of conditioner

Fig .6 SEM images of various hair treated with conditioner

Asian Caucasian African

 

Fig .7 X-ray spectroscopy of various hair treated with conditioner

From the SEM images we can not observe big differece between treated and unreated hair, since I just put the hair in the conditioner for one day, this result is not out of expection. From the X-ray sepectroscopy, we detect an increasing amout of silicon after conditioner treating. Since the Asian hair uses this conditionerr frequently, it even contains silicon before dipping in the conditioner. Based on the above observation, I think the main effect of conditioner is to leave silica on the hair surface especially in the scales, the existence of silica will provide an desirable smooth feel.

Conclusions

Although my studies were performed with few samples, the results on the morphology, and composition of hair are consisten with previously reportd results in the literature.

Acknowledgments

I wish to thank Brian McIntyre for his help and guidance through out this project.

 

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